Certain phosphorus-containing organic compounds are known to be fire retardant and heat resistant when mixed with or incorporated chemically in polymers. However such mixtures degrade the polymers. Many instances of polymers are known in which phosphorus and nitrogen atoms are alternatively linked into a long linear polymer chain with various substituents appended onto the P atoms. These polymers are used in specialized heat resistant elastomers for example. These linear chains have been shown to degrade thermally to form the cyclic phosphazene ring indicating the greater stability of this latter structure. In accordance with the present invention phosphorus is incorporated in the polymer molecules in the form of the cyclic phosphazene ring.